Nitrogen (N) fertiliser management is increasingly important in sugarcane production as imperatives to reduce environmental impacts of N escalate. In this paper we report testing of a new concept for N management in sugarcane, the N Replacement system. This system relies on soil N cycling to 'buffer' differences in crop N needs and N fertiliser supply to individual crops, and aligns N applications with actual cane production over the longer-term rather than potential production. In 11 experiments, conducted in a wide range of environments over two to five crops, cane and sugar yields in the N Replacement treatment were similar to those achieved with the farmers' conventional N management, with a trend over successive crops for yields to increase relative to conventional management. At sites where experiments ran for at least 4 years, this trend resulted in cumulative sugar yields being higher in the N Replacement treatment. Average N applications were 35% lower in the N Replacement treatment, and N lost to the environment was estimated to be ∼50% lower. Soil N 'buffering' was adequate to maintain sufficient N supply to crops even when yields were up to 30% greater than expected. Thus, it is not necessary to align fertiliser applications to potential sugarcane yields, which are rarely achieved in practice. Our results show that the ecologically-based N Replacement system has promise to deliver superior environmental outcomes without significantly reducing production of sugarcane, and potentially other semi-perennial crops, in the tropics and subtropics. Further evaluation of the system will be beneficial, and there is scope for determining more sitespecific values of parameters in the system. However, care must be taken to evaluate the system over sufficient time frames (e.g. >2 crops) so that productivity improvement trends in the N Replacement system can be expressed.

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ws Declaration of orginality:The work presented in this thesis is, to my best knowledge and beUef, original and my own work. References to others are acknowledged in the text and contributions of others noted in the statement of contributions by others. This material has not been submitted, either in whole or in part, for a degree at the University of Queensland or any other University. Ian Maxwell Biggs Acknowledgments:This project was funded by a PhD research scholarship from the Sugar Research and Development Corporation.I wish to deeply and sincerely thank Professor Christa Critchley for her rescuing this project when it was all but sunk. Her enthusiasm for a project outside her specialist field and when she akeady had a heavy workload has motivated me to completing this thesis.Special thanks to my other supervisors Professor George Stewart, Dr John Wilson, and Dr Hock Ng for their advice, encouragement, and proofreading time and skills.Thank you to Peter Thorbum, Brian Keating and Ian ValUs, CSIRO Tropical Agriculture, for sharing their thoughts on sugarcane agriculture, amino acids in sugarcane and soil nitrogen profiles under cultivated sugarcane. I would like to thank CSIRO in general for the assistance in completing this thesis on a part-time basis while maintaining my fulltime workload.Many thanks to the members of the old 'Club Metab', Susanne Schmidt, Peter Erskine, Vicki Cramer, Geoff Woodall, Scott McNeil, Nicole Robinson, Marcos Adair, Tanuwong Sangtiean, Xinhua He, Gordon Moss, Rosemary Goodall, and Jan Stewart. The many coffees, chats, and beers all helped for a full and enjoyable time as a PhD student.To the staff of the CSIRO Cunningham Library, Patrick, Robyn, Kaye, Kerry, and Nettie, thankyou for your assistance in literature searching and tracking down those elusive references. A thank you to the staff of the University of Queensland Biological Sciences Library for their assistance in the early years of this project. To the general and administrative staff of the Department of Botany, University of Queensland, thankyou for assistance in glasshouse experiments, equipment maintenance, photocopying and the supply of general materials for labs, and general administrative requirements.Warm and special thanks to my family for their support and love throughout this ordeal. Especially Lisha for her unfailing support and love, even when I was firustrated, cranky and generally angry about everything while writing the words in this book. Jeimifer and Colin for reading thesis draft and their comments and advice, and of course thank you Mum. u statement acknowledging the contribution of others:Brian Keating, Vic Catchpoole, and Ian ValUs (CSIRO Tropical Agriculture) established an experimental site at Bundaberg ("Schulte" site, Chapter 2) and allowed me access to the site for sampling as well as providing samples collected at the site from previous years. Graham Kingston (Bureau of Sugar Experiment Stations (BSES), Bundaberg) provided access to the Yandina site and historical samples from that site.Grah...